The present invention relates to Raman amplifiers and to gain control of these amplifiers.
A typical optical communication system utilizes a plurality of discrete amplifiers situated between long lengths (more than 5 km) of transmission fiber. The discrete amplifiers are usually Erbium doped fiber amplifiers also referred to as EDFAs. The distance between the Erbium doped fiber amplifiers is often referred to as the xe2x80x9cspan lengthxe2x80x9d. Typical span lengths are 25 to 100 kilometers. As the communication signal travels through the transmission fiber between two Erbium doped amplifiers, the signal attenuates and the Erbium doped amplifiers boost up the intensity of the signal.
It has been known that an optical communication system can utilize the distributed Raman fiber amplifiers in conjunction with the Erbium doped fiber amplifiers. The distributed Raman fiber amplifiers utilize transmission fiber as their gain medium. When distributed Raman fiber amplifiers are utilized in conjunction with the Erbium doped fiber amplifiers, the number of spans in an optical communication system can be increased by a factor of 2 or more. In addition, the span length between the Erbium doped amplifiers can be significantly increased. The gain performance of the Raman amplifier depends on various transmission fiber properties, such as the pump light absorption, the effective area of the fiber, and the Raman gain coefficient.
These properties of transmission fibers may vary between different fiber types and also due to the manufacturing variances within a single fiber type. Thus, given the same amount of pump power, different spans of the same length may provide different Raman gains.
The Raman gain for a small signal and a single pump wavelength is given by:   Gain  =      exp    ⁡          [                                    g            R                                              A              eff                        ⁢                          α              P                                      ⁢                  (                      1            -                          exp              ⁡                              (                                                      -                                          α                      P                                                        ⁢                  L                                )                                              )                ⁢                  P          P                    ]      
where gR is the Raman gain coefficient, xcex1p is the pump absorption, Aeff is the effective area, L is the fiber length and Pp is the pump power. Therefore, if the parameters Aeff, xcex1Por gR vary from span to span (or from fiber to fiber within the span), the Raman gain will be different. The small signal is a signal that is at least 10 times and preferably 100 times smaller than the total pump power to be provided by the pumps. It is a non-depleting signalxe2x80x94i.e., does not change the optical pump power distribution (along the length of the fiber).
Optical communication systems are designed to have a predetermined gain between spans of fiber. If the amount of gain provided by the distributed Raman fiber amplifier differs from the required (predetermined) gain, the signal input power into the EDFA is different from the input power that this EDFA was designed for and, the gain spectrum provided by the EDFA tilts. This problem is then further multiplied by the subsequent distributed Raman fiber amplifiers and the subsequent EDFAs.
According to one aspect of the present invention a Raman fiber amplifier includes: a transmission fiber; at least one optical pump providing optical pump power to the transmission fiber; at least one pump power detect or detecting optical pump power; at least one signal detector detecting signal power propagating through the transmission fiber. The Raman fiber amplifier also includes a controller that adjusts the pump power provided by the pump, to adjust gain or signal power provided by this Raman fiber amplifier.
According to an embodiment of the present invention, a method of controlling gain in a Raman fiber amplifier comprises at least two optical pumps supplying power in wavelength xcex1 and at least two optical pumps supplying power in at least one other wavelength xcexi, includes the steps of: (i) determining pump driving currents Ci for each the optical pumps so that the amount of power provided by the same wavelength pumps is about same; (ii) determining the ratios of the pump driving currents with respect to one another; (iii) driving the optical pumps with the pump driving currents, while maintaining these ratios; (iv) determining total pump powers Pi to be provided in each of the pump wavelengths; (v) adjusting the driving currents Ci proportionally, with respect to one another, to provide the pump powers Pi.
Additional features and advantages of the invention will be set forth in the detailed description, which follows. In part it will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein. Including the detailed description, which follows the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.